1. Field of the Invention
The present invention relates to a hologram replicating device, a hologram replicating method that uses contact printing and a replicated image recording medium.
2. Description of the Related Art
Holograms which provide stereoscopic images are used for authentification of, for example, credit cards and ID cards. Embossed holograms having an interference film recorded by providing surface unevenness are now often employed. Embossed holograms, however, are easy to forge. Lippmann holograms, in which an interference film is recorded as differences in refractive index inside the film, are very difficult to forge. This is because highly advanced technology is necessary to produce a recorded Lippmann image and the recording material is not easily available. A Lippmann hologram can be produced by using an actually captured hologram obtained by irradiating an object with a laser or using a holographic stereogram recorded on the basis of parallax images observed from multiple viewpoints.
Schematically, a process for producing a Lippmann holographic stereogram includes a content producing step for, for example, acquiring images and editing the acquired images, a hologram master generating step and a replicating (i.e., mass-producing) step. The image is acquired by image capturing or generated through computer graphics. Each of the acquired images acquired in the image editing step is converted into a strip-shaped image by, for example, a cylindrical lens. A hologram master is produced by sequentially recording interference fringes of object light of the images and reference light on a hologram recording medium as strip-shaped element holograms. The hologram recording medium and the hologram master are brought into close contact with each other and irradiated with a laser beam, whereby a hologram is replicated.
In this hologram, for example, image information acquired by sequentially performing image capturing from different viewpoints along a transverse direction is sequentially recorded as strip-shaped element holograms along the transverse direction. When this hologram is observed by a viewer with both eyes, two-dimensional (2D) images observed by the left and right eyes slightly differ from each other. This phenomenon causes the viewer to experience parallax, and a three-dimensional (3D) image is reproduced.
If the strip-shaped element holograms are sequentially recorded as described above, a horizontal-parallax-only (HPO) holographic stereogram having parallax only in the horizontal direction is generated. The HPO holographic stereogram can be printed in a short time and with high recording quality. Vertical parallax can also be employed in the recording method. Holograms with parallax in both horizontal and vertical directions are called full parallax holograms.
As described above, a Lippmann hologram is difficult to forge compared with an embossed hologram and thus is suitable for use in verifying authentification of, for example, credit cards and ID cards. If additional information, such as a serial number and identification (ID) information, can be recorded, forgery becomes more difficult. Since it is inefficient to generate holograms one at a time using a printer, replication of plural holograms by contact printing has been proposed.
In a replication device of the related art which employs contact printing as illustrated in FIG. 23, a laser beam (i.e., S polarized light) from a laser light source 200 is expanded by a spatial filter 203 and is incident on a collimation lens 204. A hologram recording medium 205 having a photosensitive material and a hologram master 206 are irradiated with parallel laser beams collimated by the collimation lens 204.
The hologram master 206 is a reflection hologram, e.g., a Lippmann hologram. The hologram recording medium 205 has a photosensitive material layer and is in close contact with the hologram master 206 directly or with a refractive index adjusting liquid (also called an index matching liquid) therebetween. An interference fringe formed by light diffracted by the hologram master 206 and an incidence laser beam is recorded on the hologram recording medium 205.
In order to make forgery of the hologram more difficult, recording of additional information in the hologram has been proposed. For example, Japanese Unexamined Patent Application Publication (JP-A) No. H11-258970 discloses recording of additional information during contact printing of the hologram.
In the device disclosed in JP-A-H11-258970, a reflection hologram master and a recording film are in close contact with each other with an optical adhesion liquid therebetween and a reflective liquid crystal device is disposed at a non-hologram area neighboring the hologram master. Additional information is displayed on a reflective liquid crystal device by a computer. The hologram master and the liquid crystal device are irradiated with a laser beam from a side of the recording film via an optical system. Reflected light (i.e., object light) and reference light from the hologram master are made to interfere with each other within the recording film so as to implement hologram recording. At the same time, the reflected light from the reflective liquid crystal device and the reference light interfere with each other within the hologram recording film and the additional information displayed by the reflective liquid crystal device is recorded in the recording film.
JP-A-2008-122670 discloses a screen switching hologram in which different 3D screens are switched between depending on the viewpoint. The disclosed hologram is recorded in two steps. In a first step, objects to be displayed on different screens are recorded on plural element hologram recording materials as holograms using reference light at the same incidence angle and the plural element holograms are arranged in parallel to form a first level hologram. Next, the object image recorded on each element hologram from the first level hologram is reproduced simultaneously. A second level hologram recording material is disposed near the reproduced object image and is recorded as a reflection or a transmission volume hologram. In reproduction of the thus-recorded hologram, different hologram images are reproduced depending on the viewpoint of an observer.